GaAs-based Vertical-cavity Surface-emitting Lasers Research Articles

An InGaAs Vertical-Cavity Surface-Emitting Laser Emitting at 1130 nm for Silicon Photonics Application

A highly strained InGaAs quantum well (QW) vertical-cavity surface-emitting laser (VCSEL) with low threshold current density, high efficiency and output power emissions around 1130 nm was grown by MOCVD. Its static characteristics at room temperature and high operation temperature were studied in detail. The 7 μm oxide aperture device exhibits a threshold current of 0.68 mA, corresponding to a threshold current density of 1.7 kA/cm2. The slope efficiency is 0.43 W/A and the maximum output power is 3.3 mW. Continuous-wave (CW) operation in the 10–80 °C temperature range is observed. The slope efficiency is almost constant at 10–80 °C. The threshold current becomes lower at high temperatures thanks to the alignment between gain peak and cavity mode. The 3 μm oxide aperture device’s lasing in single mode with the RMS spectral width of 0.163 nm and orthogonal polarization suppression ratio (OPSR) is ~15 dB at 25 °C. The small-signal response analysis indicates that reducing the parasitics of the device and refining the fabrication process will improve the dynamics response characteristics. These results indicate that the 1130 nm GaAs-based VCSEL with highly strained InGaAs QWs is expected to be used as source for silicon photonics.

Improved optical efficiency of GaAs-based infrared vertical-cavity surface-emitting laser enabled by combining a metallic reflector and a Bragg reflector

The use of coupled reflectors was investigated to improve the device performance of GaAs-based 860 nm vertical-cavity surface-emitting lasers (VCSELs). Here, the combined reflector was fabricated by coupling a metallic reflector (Ag) and a distributed Bragg reflector of an epitaxial structure. The performance parameters of the VCSEL, such as light-emitting efficiency, operational voltage, and power consumption, were dramatically improved by using a combined reflector. As a result, infrared GaAs-based VCSELs with combined reflectors could show superior light-emitting performance and reduced operational threshold voltage characteristics compared to conventional VCSELs.

Oxide-confined GaAs-based vertical-cavity surface-emitting laser: Measurement and modeling of the strain field

The strain field of the emitters of one unstressed and two electrically stressed oxide-confined GaAs-based vertical-cavity surface-emitting lasers has been analyzed. The components of the strain tensor in the optical aperture, in the quantum wells, and at the oxide edge of the lamellas have been measured by nano-beam precession electron diffraction and geometrical phase analysis. The measurements have been used to validate the simulations based on the finite element method (FEM) of the mechanical behavior of the emitters. The FEM model is used to quantify the strain caused by the lattice mismatch of the GaAs-based epitaxial layers and the volume variation of the Al 98Ga 2As layer due to selective oxidation. The first is calculated as a function of the misfit strain and corresponds very well to the measured values. The latter is calibrated assuming that the thickness of the oxide is expanded by 4 % in both the unstressed sample and the stressed sample with dislocations. Instead, it is contracted by 8 % for the sample where the stress test has caused further oxidation.

Impact of cavity design on the modulation properties of a GaAs-based VCSEL

GaAs-based vertical-cavity surface-emitting lasers (VCSELs) are commonly used as light sources for optical interconnects in short-range systems. Here, we analyse the impact of the cavity Q factor and the number of quantum wells in the active region on the small-signal modulation response of a GaAs-based VCSEL. By tuning the reflectivity of the mirror it is possible to obtain flat response curves with a high −3 dB bandwidth. It is also beneficial to increase the number of quantum wells in the active region.

795-nm high-temperature and high-power operating vertical-cavity surface-emitting laser and application in atomic gyroscope

Single-transverse mode vertical-cavity surface-emitting lasers (VCSELs) are preferable optical sources for small low-power atomic sensors, including chip-scale atomic clocks, magnetometers, and gyroscopes.When VCSEL is used as the pump source of nuclear magnetic resonance gyroscope, it is required to have high single-mode output power. Oxide aperture diameter must be sufficiently small (< 4 µm) in a conventional oxide-confined VCSEL to support the fundamental mode alone. However, high series resistance (typically > 200 Ω for GaAs-based VCSEL) from the small aperture limits its output power and reliability due to excessive current-induced self-heating and high current density. It is a very attractive idea to achieve high power operation of an intrinsic single mode VCSEL based on a large oxide aperture by means of epitaxial structure design without introducing additional process steps. Transverse optical confinement in oxide-confined VCSELs crucially depends on the thickness of oxide layer and its position relative to standing wave. Modifying the structure reduces the overlap between the oxide layer and the standing wave as well as the difference in effective refractive index between core and cladding of the VCSEL, thereby reducing the number of transverse modes andincreasing the mode extension beyond oxide aperture. A 795-nm VCSEL is designed and fabricated based on this concept. A cavity structure of VCSEL with gain-cavity detuning of ~10.8 nm at room temperature is adopted in this paper. The effective refractive index and the standing wave distribution of the VCSEL are calculated, and the position of the oxide layer in the epitaxial structure of the VCSEL is optimized according to the standing wave distribution. Finally, the structure with low effective refractive index difference is obtained. The proposed device achieves high single-mode operation of 4.1 mW at 80 ℃, SMSR of 41.68 dB, and OPSR of 27.4 dB. The VCSEL is applied to a nuclear magnetic resonance gyroscope (NMRG) system as pump source due to its excellent device performance, and satisfactory test results are obtained. This paper presents a new method of designing single-mode high power VCSEL and its feasibility is also demonstrated through experimental results.

Mechanical strain mapping of GaAs based VCSELs

We report an investigation of the strain field in mesa structures for oxide-confined vertical-cavity surface-emitting lasers (VCSELs) using the polarization-resolved micro-photoluminescence (PL) measurement of the degree of polarization (DOP) at room temperature. The DOP of the PL is correlated with the spatial distribution of the embedded anisotropic strain in a VCSEL structure. Measurements normal to (100) surfaces of the samples and from (110) cross-sectional planes were performed. The effect of two processes required in the fabrication of GaAs-based VCSELs was studied: the plasma etching of the P-doped distributed Bragg reflector and the wet oxidation process used to control current flow and lateral optical confinement. The DOP method allows very sensitive measurements of the mechanical strain (on the order of 10−5) accumulated in VCSEL devices even during different steps of the fabrication process.

Improving Performance of GaAs-Based Vertical-Cavity Surface-Emitting Lasers by Employing Thermally Conductive Metal Substrate

We investigated the effect of the conducting substrate on the performance of GaAs-based VCSELs, where the substrates were 230 μm-GaAs (reference), 10 μm-GaAs/metal, and 0.5 μm-GaAs/metal. The VCSELs with the 10 μm- and 0.5 μm thick GaAs/metal-substrates produced higher light output power than the reference. For example, the thin GaAs/metal substrate samples showed 16.3%–16.7% higher light output power at 3.0 A than the reference. It was shown that the thin GaAs samples produced 12.2%–14.0% higher power conversion efficiency at 3.0 A than the reference. At a high current region, the metal-substrate samples yielded lower junction temperature than the reference, namely, the thin GaAs samples gave 42 °C–47.4 °C lower junction temperature at 2.0 A than the reference. Further, the thin GaAs samples revealed better light output degradation characteristics than the reference. For instance, the light output of the reference was degraded by 30.2% at 85 °C, whereas the thin GaAs samples were degraded by 20.1%–20.5%. Near-field images and emission profiles demonstrated that the metal-substrate samples suffered from no damage incurred during the VCSEL fabrication process.

GaN-based vertical cavity surface emitting lasers with lateral optical confinements and conducting distributed Bragg reflectors

We aimed to further improve performances of GaN-based vertical cavity surface emitting lasers (VCSELs) by applying a combination of conducting distributed Bragg reflectors (DBRs) and lateral optical confinement structures simultaneously, generally used in GaAs-based VCSELs. Si-doped conducting AlInN/GaN DBRs and buried SiO2 apertures were adopted in the GaN-based VCSELs. By comparing the VCSELs and micro LEDs to those with undoped non-conducting DBRs, we found that lower device resistances and more uniform lateral current distributions were obtained with the conducting DBRs. At the same time, the maximum light output power of 2.6 mW was observed from the VCSEL with the conducting DBR while 4.4 mW was obtained from the VCSEL with undoped DBR. Inferior characteristics of a GaInN quantum well active region was found on the Si-doped conducting DBR.

Continuous-Wave Operation of GaAs-Based 1.5-μm GaInNAsSb VCSELs

We demonstrate the first electrically-pumped, GaAs-based vertical-cavity surface-emitting lasers (VCSELs) emitting in the 1500 nm range that operate in continuous wave mode at and above room temperature. The VCSELs employ a dilute-nitride GaInNAsSb/GaNAs multiple quantum well active region, two n-doped AlGaAs/GaAs distributed Bragg reflectors and a GaAs tunnel junction. Continuous-wave single-mode lasing was observed up to 50 °C with room-temperature threshold current densities below 3 kA/cm2. Small signal measurements indicate that dilute nitride GaInNAsSb/GaNAs active regions are capable of high speed modulation.

GaN-based vertical-cavity surface-emitting lasers using n-type conductive AlInN/GaN bottom distributed Bragg reflectors with graded interfaces

A 4.0λ-cavity GaN-based vertical-cavity surface-emitting laser (VCSEL) using an n-type conductive AlInN/GaN bottom distributed Bragg reflector (DBR) showed a light output power of 1.8 mW with a low differential resistance of 90 Ω. In order to obtain low resistive AlInN/GaN DBRs, 5 nm AlGaInN graded interfaces were introduced. A 40-pair AlInN/GaN DBR with the graded interfaces showed a peak reflectivity over 99.8% and a series resistance of 17 Ω. At the same time, the maximum light output power of a 1.5λ-cavity GaN-based VCSEL with the conductive DBR was only 0.03 mW due to a high thermal resistance of 2700 K W−1 caused by a high thermal resistivity of AlInN. We also conclude that a short cavity typically used in GaAs-based VCSELs is not appropriate in GaN-based VCSELs.

Use of a rough GaP layer to improve light efficiency of 860-nm GaAs-based vertical-cavity surface-emitting laser

Abstract The effects of using a rough GaP layer to improve the light efficiency of 860-nm GaAs vertical-cavity surface-emitting laser were investigated. The light efficiency was significant increased by enlargement of current flow path caused from use of GaP layer grown on the top distributed Bragg reflector (DBR) of VCSEL. Furthermore, it was found that improved light efficiency significantly could be further increased by optimizing the surface morphology of the p-GaP layer fabricated in limited surface lighting area of VCSEL. Various surface morphologies could be obtained easily by controlling the RF power of an ICP dry etching. The highest output power of 34 mW was obtained from the VCSEL with a rough GaP layer fabricated at a RF power of 40 W introduced with BCl3 gas, showing a noticeable increase of ∼113% than a conventional one (16 mW). This result clearly insists that light efficiency of 860-nm GaAs based VCSEL could be increased effectively by using the rough GaP layer.

Compact Fast Optode-Based Probe for Single-Photon Counting Applications

We present a compact standalone optode probe integrated with a pulsed vertical cavity surface emitting laser (VCSEL) and a fast-gated single-photon counting module. The single-photon counting module is fabricated in the 0.35- $\mu \text{m}$ HV CMOS technology and coupled to a silicon-photon avalanche diode (SPAD). The integrated 10- $\mu \text{m}$ diameter SPAD has a photon detection efficiency of 5% at a wavelength of 850 nm and a very low dark count rate of 60 cps at room temperature. A compact and very high modulation rate top-emitting GaAs-based VCSEL chip is used to deliver pulsed picosecond light. High-intensity short-time reflections, occurring due to the close proximity of the light source and detector, are avoided by fast-gating of the detector, which enables the detection of photons in some specified time-windows. A full characterization of the module is done without any cooling, testing both its electric and optical properties. The compactness of the module paves the way for its utilization in a wide variety of single-photon counting applications.

1060 nm VCSELs for long-reach optical interconnects

Abstract Reach extension of high capacity optical interconnects based on vertical-cavity surface-emitting lasers (VCSELs) and multimode fibers (MMFs), as needed for large-scale data centers, would benefit from high-speed GaAs-based VCSELs at 1060 nm. At this wavelength, the chromatic dispersion and attenuation of the optical fiber are much reduced in comparison with 850 nm. We present single and multimode 1060 nm VCSELs based on designs derived partly from our high-speed 850 nm VCSEL designs. The single-mode VCSEL, with a modulation bandwidth exceeding 22 GHz, supports back-to-back data rates up to 50 Gbps at 25 °C and 40 Gbps at 85 °C under binary NRZ (OOK) modulation. Using mode-selective launch, we demonstrate error-free 25 Gbps transmission over 1000 m of 1060 nm optimized MMF. Higher data rates and/or longer distances will be possible with equalization, forward-error-correction, and/or multilevel modulation.

Increased wavelength VCSELs reach out

Work from Sweden and the US is extending the reach of low-cost laser and fibre technology to meet the growing needs of optical interconnects in the massive data centres that underpin our Internet-enabled world. Team member Ewa Simpanen holding up a set of VCSEL chips in the cleanroom at Chalmers University of Technology. VCSELs (Vertical-cavity surface-emitting lasers) are power efficient semiconductor lasers that can be produced in high volumes at low cost. They can also be directly modulated at high speed when biased at low currents. All of this makes them the preferred light source for the short-reach optical interconnects used in data centres and high-performance computing systems. In these applications, VCSELs are used with high modal bandwidth multimode fibre (MMF) to relax the laser-to-fibre alignment tolerance, which also reduces costs associated with assembly and packaging. With the development of cloud-based applications and Internet-delivered services, data centres operated by service providers like Facebook and Google are developing into very large facilities. These ‘mega’ data centres need optical interconnects that can reach beyond 1 km. This cannot be supported by current VCSEL-MMF technology owing to the large chromatic dispersion and attenuation in MMF at the standard wavelength – 850 nm. At longer wavelengths, dispersion and attenuation are much lower, allowing greater reach. This is the need that the team, from Chalmers University of Technology in Sweden, and Hewlett Packard Enterprise (HPE) in the US, are working to meet. Team member Ewa Simpanen told us, “The purpose of this work is to develop high-speed, longer wavelength those while still using the GaAs-based material systems as it enables the fabrication of VCSELs which are superior to those based on other material systems in terms of speed, efficiency, manufacturability, and cost.” Currently, large-scale data centres use single-mode fibres and long-wavelength silicon photonic transceivers to cover the needed distances. With longer wavelength GaAs-based VCSELs, it would be possible to build transmitters for long-range interconnects that are as power and cost efficient as those used for shorter reach interconnects. In this issue of Electronics Letters, the Chalmers and HPE team present a VCSEL design operating at 1060 nm. This high-speed, single-mode, low-resistance VCSEL can support bit-rates up to 50 Gbit/s while dissipating no more than 100 fJ of energy per bit; making it very energy efficient – on par with the state-of-the-art at 850 nm. “We have limited the wavelength extension to 1060 nm since the long-term reliability of GaAs-based VCSELs may be compromised if extending the wavelength beyond ∼1100 nm,” explained Simpanen. At 1060 nm, the fibre chromatic dispersion is reduced by 70% and attenuation by 50% compared to 850 nm. Using a single-mode VCSEL the narrow spectral width of the laser further reduces the impact of chromatic dispersion. With the single-mode VCSEL properly aligned to the MMF, only the fundamental mode of the fibre, or the lowest order mode group, is excited, which also reduces the impact of modal dispersion. “In these conditions, the fibre ideally acts as a pure attenuator without degrading the signal quality. All this together should enable data transmission distances compatible with what is required in mega data centres,” said Simpanen. The results reported in this issue deal with the modulation capacity of the VCSEL itself. Since then, the team have started investigating the data transmission capacity of optical interconnects using the VCSEL together with MMF designed for high modal bandwidth at 1060 nm and so far the results are very promising, according to Simpanen. “This prototype fibre is fabricated at and provided by Corning. In a first preliminary experiment, we have transmitted data at 25 Gbit/s over 1 km with some margin. We believe that higher data rates and/or longer transmission distances are possible with further optimisations. We will also investigate the alignment tolerances under mode selective launch as they are of utmost importance for practical implementations.” A microscope image of an array of VCSEL devices on chip; newly fabricated and ready for characterisation. Looking further ahead, the team think that continued engineering of conventional VCSEL designs and the implementation of more exotic VCSEL concepts for further significant speed improvements will play a big part in developing much higher capacity interconnects. Combined with electronic compensation techniques, such as equalisation and digital signal processing, higher order modulation formats, and more advanced forms of spatial and wavelength division multiplexing, they believe such lasers could eventually achieve aggregate interconnect capacities approaching 100 Tbit/s.

Many-valley electron transport in AlGaAs VCSELs

Carrier transport in GaAs-based vertical-cavity surface-emitting lasers (VCSELs) is investigated by means of an in-house multiphysics code, with particular emphasis on the description of many-valley effects in the conduction band of AlGaAs barriers. These effects, which are revealed to have a significant impact on the overall VCSEL performance, are accounted for by an effective density of states obtained with a closed-form model. This description has been included in a simplified simulation framework, where most of the distributed Bragg reflector pairs are replaced by an equivalent homogeneous layer. This leads to a major reduction of the computational cost, especially important in view of the computer-aided design of 3D devices.

Effect of Relief Aperture on Single-Fundamental-Mode Emission of 1.3-$\mu $ m GaInNAs GaAs-Based VCSELs

This paper investigates methods of suppressing higher order transverse modes in a GaInNAs/GaAs quantum-well GaAs-based vertical-cavity surface-emitting diode laser, with the aid of a 3-D self-consistent model. The inverted surface relief design, which creates the antiphase condition, proved efficient at suppressing higher order transverse modes, in spite of unfavorable thermal effects.

A study on the law of oxidation rate in GaAs-based VCSELs

In order to accurately control the oxidation aperture in high power vertical cavity surface emitting lasers (VCSELs), the selective oxidation process is studied with experiments. Selective oxidation experiments are carried out upon the simulate wafer of VCSELs at different temperature. Oxidation products are examined at different oxidation depths of oxidation layer and each component content is analyzed. The results of the experiments are analyzed with the kinetics of thermal diffusion. The analyzed results show that the concentration of oxidant is e exponentially declined with increasing depth of oxidation in high-power VCSELs. The oxidation depth stability and precision can be improved by lowering the oxidation temperature and prolonging the oxidation time.

The Influence of Imperfections and Absorption on the Performance of a GaAs/AlO $_{\rm x}$ High-Contrast Grating for Monolithic Integration With 980 nm GaAs-Based VCSELs

This paper discusses the influence of fabrication imperfections and absorption on high contrast grating (HCG) reflectivity and reveals that HCGs show considerable tolerance to both factors. A fully vectorial optical numerical model was used to design an HCG for use as the top mirror of a 980 nm GaAs-based vertical-cavity surface-emitting laser. Parameters providing a high degree of reflectivity were determined with special attention to optimizing the low refractive index layer.

Diode pumped 850 nm vertical-cavity surface-emitting laser

Very little, if any, has been published on optically pumped 850 nm vertical-cavity surface-emitting lasers (VCSELs), particularly for doped structures. This paper investigates GaAs-based VCSELs which have not been optimized for optical pumping work. Characterisation was carried out for both pulsed and continuous wave (CW). Pulsed operation causes a lower rise in temperature, thus postponing the onset of thermal rollover, and allowing the device to be operated at higher powers. A threshold of ∼160 kW/cm 2, and single mode output with incident power density of up to 225 kW/cm 2 were obtained. From the simulation work done, it has been observed that for optically pumped VCSELs, at higher pump density, there was faster turn on and higher output power, and that dilute nitride active material give better output performance compared to GaAs.

Emerging applications for vertical cavity surface emitting lasers

Vertical cavity surface emitting lasers (VCSELs) emitting at 850 nm have experienced explosive growth in the past decade because of their many attractive optical features and incredibly low-cost manufacturability. This review reviews the foundations for GaAs-based VCSEL technology as well as the materials and device challenges to extend the operating wavelength to both shorter and longer wavelengths. We discuss some of the applications that are enabled by the integration of VCSELs with both active and passive semiconductor elements for telecommunications, both in vivo and in vitro biosensing, high-density optical storage and imaging at wavelengths much less than the diffraction limit of light.